At the present time special attention is paid in several different countries to the satellite-based localisation of mobile stations and to various services relating to localisation. For example, in the United States of America the authorities have established a time limit, by which mobile stations must include a localisation system, with the aid of which a call received by an emergency station can be localised with an accuracy of approximately one hundred meters. Mobile stations with an integrated GPS-based (Global Positioning System) localisation system are already commercially available.
In practice, such a GPS receiver is integrated into the mobile station, which receives signals transmitted by satellites and which calculates its location with the aid of information contained in the signal. The GPS satellite transmits two different carrier wave frequencies. Two pseudo random signals (a C/A code and a P code intended for military use) and a navigation message are modulated into the carrier waves. The code messages do not contain actual information, but they are pseudo random sequences including a +1 and a −1 space and they are used for modulating the carrier wave. In addition, such a navigation message is modulated into the carrier wave, which contains information related to determination of the location. The length of its frame is 1500 bits, and the frame is divided into five sub-frames, all of an equal length. The frame includes e.g. clock correction information, check bits, information on the age of data, on the orbits of satellites and on the location of satellites at different times.
Continuous three-dimensional localisation requires no less than four simultaneously visible satellites. The determination of location is based on a determination of the distance between receiver and the group of satellites. The location can be determined with the aid of at least three satellites, whereby the fourth is mainly needed to eliminate deviations caused by clock errors.
Since the signal transmitted by a satellite is accompanied by information on the time of departure of the signal, the receiver is able to calculate the signal's transit time from the difference between its own clock time and the satellite's clock time. The distance of the satellite can hereby be determined, by multiplying the signals transit time by the signal's velocity, that is, by the velocity of light.
There are several different kinds of GPS signal receivers. The differences relate e.g. to whether the receiver is a single-frequency receiver or a two-frequency receiver, whether the receiver observes several satellites constantly or each satellite alternately, and what kind of code it identifies. The number of channels is an important feature of the receiver. This means how many satellites the receiver is able to observe simultaneously. The channels may be implemented either at equipment level or by software.
To allow as accurate a time determination as possible, the satellite includes a cesium clock, with which it takes more than two hours for one nanosecond to accrue. The clock error is taken into account by transmitting the clock error information coded into the signal. The clock of the receiver is usually a quartz crystal, which is less accurate than the atomic clock. With the quartz crystal it takes only a second for a nanosecond error to accrue, and its errors are more difficult to foretell. The clock error can be corrected, either by having the receiver update its clock when the error has grown to a certain predetermined value, or by using an external stable source of frequency.
Localisation of a mobile station does not require synchronised networks. Base transceiver stations are independent of each other, so the frames transmitted by them are not in synchrony with each other. In other words, base transceiver stations are not aware of the starting time of the frames they transmit. This drawback is due to the fact that no reliable reference clock has been available so far for implementing synchronisation. In a non-synchronised network, the various frames will cause such interference to one another, which may at worst destroy several time slots. FIG. 1 illustrates the present situation, where base transceiver stations located in the cells (cell 1-cell 5) of a cellular network transmit frames over the air interface in a non-synchronised network. As can be seen in the figure, the time slots TS1-TS8 of the frames are not mutually matched. It is impossible to know which frequencies are interfering with what, and the base station controller is unable to calculate the mutual timings of the frames transmitted by the cells.
The above-mentioned drawback can be reduced considerably, if the frame transmissions to be transmitted at the air interface are synchronised. In a synchronised network, no more than one time slot is lost in the worst case. FIG. 2 illustrates broadcasting of frames in a synchronised network. When the frames are exactly matched, e.g. allocation of channels can be done exactly on a time slot basis.
For the localisation function, measuring equipment will be installed in the future in the mobile network at the locations of base transceiver stations for localisation of mobile stations. In accordance with the present invention, these pieces of equipment can be utilised also for synchronisation of frame transmissions at the air interface in a mobile network. The solution is economically advantageous, because no separate equipment is required for the synchronisation only.